When the bodies of Christian adventure camp counselors Lindsay Cutshall, 22, and her fiancé, Jason Allen, 26, were found on a remote stretch of beach in Northern California in August of 2004, police immediately began searching for their killer.

But they had few leads to go on.

What they did know is that before their deaths, Lindsay and Jason had taken a break from their jobs as counselors at Rock-N-Water, an outdoor Christian adventure camp in Northern California, to go away for the weekend, first to San Francisco and then up the coast to Sonoma County.

On the night of August 14, or early that next morning, the couple was killed while they were sleeping on a secluded stretch of Fish Head Beach, in Jenner, where they'd stopped for the night.

On Monday night, the couple's murders are examined in-depth on the next episode of People Magazine Investigates, at 10 ET/9 CT on Investigation Discovery and streaming on discovery+. (An exclusive clip is shown below.)

In the episode — Jenner Beach Murders — Lt. Tim Duke of the Sonoma County Sheriff's Office talks about how investigators painstakingly tracked down clues in the perplexing case — and how a key piece of information came from Jason's autopsy.

"A piece of a fragmented bullet was retrieved from Jason's head that gave us some ballistic information on the bullet itself and the rifle that fired it," Duke says in the episode.

"By examining the bullet fragments," PEOPLE Editor-in-Chief Dan Wakeford says in the episode, "ballistic experts determine that the rifle used was actually a unique gun — the lever-action .45-70 Marlin rifle."

That gave investigators a huge advantage in their quest to find out who stole the lives of the promising young couple, who'd been engaged for two years and planned to wed that September.

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Over the years, Sonoma County detectives continued to investigate the brazen double-murder, following up on many leads — and pursuing what turned out to be many dead-ends.

Would they ever find the couple's murderer?

For more on this story, watch Jenner Beach Murders on People Magazine Investigates, on Monday, March 29, at 10 ET/9 CT on Investigation Discovery and streaming on discovery+.

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